1. Field of the Invention
The present invention generally relates to a lens-shifting device, and more particularly, to a projection lens-shifting device having a high positioning resolution and a self-locking function.
2. Description of Related Art
At present, the lens of most projectors are fixed. Therefore, the user shifts the position of image projected on the screen only by adjusting the relative position and angle between the projector and the screen. Therefore, to project a suitable image on the screen, the user must spend a lot of time and effort to position the projector such that the projector is moved to a suitable location or adjusted to a suitable angle relative to the screen.
After fixing the location of the projector, if the image projected on the screen needs to be raised a little higher, the user is required to adjust the height adjusting device disposed on the front leg of the projector so as to modify the tilt angle of the projecting direction. However, because the angle between the projection lens and the screen has been changed, the image projected on the screen is distorted and changes, for example, from a rectangular shape to a trapezoidal shape.
To resolve the foregoing problem, a lens-shifting mechanism for a projector has been developed to facilitate the adjustment of the lens when the projector is in a static state and achieve the purpose of adjusting the image projected on the screen.
FIG. 1 is a perspective view showing a conventional mechanism for shifting a projection lens. As shown in FIG. 1, the projection lens-shifting mechanism 1 mainly includes a driving wheel 11, a worm gear 12, two fixing rods 13, 14 and a movable plate 16. Furthermore, one side of the movable plate has a rack 161. The projection lens 18 is disposed on the movable plate 16. The movable plate 16 is movably mounted on the fixing rods 13 and 14. The worm gear 12 is disposed at one end of the driving wheel 11, and the driving wheel 11 and the worm gear 12 are co-axial. Moreover, the fixing rod 13 passes through the driving wheel 11 and the worm gear 12, and the driving wheel 11 and the worm gear 12 are capable of rotating relative to the fixing rod 13. When the driving wheel 11 is driven to rotate, the worm gear 12 also rotates. The rotation of the worm gear 12 drives the rack 161 engaged therewith so that the movable plate 16 moves relative to the fixing rods 13 and 14 in the directions as indicated by the double arrow in the figure. Hence, the purpose of shifting the position of the projection lens 18 is achieved.
However, the worm gear 12 and the rack 161 may not be tightly engaged due to gaps created in the design or produced by the assembling process. Therefore, when the projector is being transported from one place to another, the movable plate 16 may slide instead of being self-locked so that the worm gear 12 and the rack 161 may not be perfectly engaged. Consequently, high resolution positioning is not provided.